Nuclear magnetic resonance spectrometer and method for measuring the nuclear magnetic resonance

ABSTRACT

A nuclear magnetic resonance spectrometer comprises a sample holder being arranged in a constant magnetic field of a predetermined direction. The sample holder comprises a rotor adapted to receive a sample under investigation. The rotor is arranged to rotate the sample about a first axis being inclined to the predetermined direction at an acute angle of, preferably, 54.7°. For heating up the rotor, a laser is provided emitting a laser beam which is directed upon a surface of the rotor. The laser beam is adjustable in intensity by means of a control device.

The present invention relates to a nuclear magnetic resonancespectrometer comprising a sample holder which is arranged in a constantmagnetic field of a predetermined direction and which comprises a rotoradapted to receive a sample, the rotor being arranged to rotate about afirst axis which is inclined to the said predetermined direction at anacute angle of, preferably, 54.7°, and means being provided for heatingup the rotor.

The present invention further relates to a nuclear magnetic resonancespectrometer comprising a sample holder adapted to receive a sample, anda laser the laser beam of which is directed upon a surface of the sampleholder.

Finally, the present invention relates to a method for measuring thenuclear magnetic resonance of a sample, where the sample is heated upduring the measurement to a temperature above ambient temperature, bymeans of a laser beam.

U.S. Pat. No. 4,201,941 describes a nuclear magnetic resonancespectrometer where a sample head comprises a sample holder which in itsturn contains a rotor adapted to a receive a sample substance. The rotorof this known nuclear magnetic resonance spectrometer is adapted torotate about an axis which is inclined relative to the longitudinal axisof the sample head and, thus, relative to the direction of thesurrounding constant magnetic field, at an angle of 54.7°, known as the"magic angle". As is generally known, the "magic angle" of 54.7° resultsfrom a term of a Legendre's polynomial according to which 3 cos² Θ-1=0.If a sample of a solid body is rotated, during a nuclear magneticresonance measurement, about an axis which is inclined relative to thedirection of the constant magnetic field by the "magic angle", then anyinteractions will be averaged out to the greatest possible degree,whereby a simplified spectrum is obtained.

In the case of the known spectrometer, rotation of the rotor is ensuredby a turbine-like arrangement. The rotor is provided for this purposewith a conical section with engraved areas configured in the manner ofturbine blades.

The conical section is received in a matching conical receiving openingof a stator which is fixed to the sample head. Inside the conicalreceiving opening, nozzles are arranged along a circumference forintroducing a propellant gas. This arrangement ensures not only aturbine drive for the rotor, but at the same time an air-cushion bearingfor the rotor in the receiving opening.

The known nuclear magnetic resonance spectrometer is further equipped,in the immediate neighborhood of the rotor, with a temperature sensorserving for monitoring the temperature of the propellant gas and, thus,of the sample.

Although this fact is not expressly mentioned in U.S. Pat. No.4,201,941, it has been known in connection with nuclear magneticresonance spectrometers of the before-mentioned type to control thetemperature of the sample via the temperature of the propellant gas,which can be achieved, for example, by heating up the compressed airwhich is to be blown into the sample head as air cushion and for drivingthe rotor.

However, this manner of proceeding is connected with the disadvantagethat a relatively large quantity of temperature-controlled propellantgas is required for controlling the temperature of the sample, and thisin particular in the case of high-temperature measurements on samples,where a correspondingly high volume of high-temperature gas will berequired. Further, it is a drawback of this method that due to thebefore-mentioned manner of controlling the temperature via thepropellant gas, all parts of the sample head which get into contact withthe gas are automatically heated up as well. This firstly leads toincreased energy consumption required for heating up the gas, andfurther results in the additional serious drawback that the entire areaof the sample head will be heated, including those parts for which suchheating-up is undesirable. This applies in particular to the receivingcoil whose thermal background noise will be greatly increased by a risein temperature. This is true in particular when the sample head isarranged in a cryostat of a superconductive magnet coil. Finally, it isa disadvantage of the described manner of controlling the temperature ofthe sample via the propellant gas that, as is generally known, theviscosity of gases, such as air, is largely dependent on temperature. Asa result of this fact, any heating-up of the propellant air may lead torotation problems as the viscosity of the air will rise considerably dueto the rise in temperature.

Another considerable disadvantage of the known temperature controlresides in the fact that the temperature of the sample must bedetermined either directly via the temperature of the heated propellantgas, or by direct measurement of the temperature in the sample head.This is extremely difficult in both cases, and in the first case evenconnected with considerable inaccuracy risks as for technical reasonstemperature measurements of the heated propellant gas can be carried outonly outside the sample head, i.e. at a considerable distance from thesample holder.

In addition, constant measurements and, if necessary, adjustments or aconstant control of the temperature would be required in this case sincethe propellant case is normally heated up by an electric heating system,and the resulting temperature is of course dependent on the initialtemperature of the incoming air, on the air flow rate, and on otherfactors. The air flow rate, for example, may vary considerably, forexample when the before-mentioned temperature-induced variation inviscosity of the propellant gas has to be compensated.

From the U.S. Magazine "Review of Scientific Instruments" (51), Vol. 4,April 1980, pp. 464 to 466, there has been known an electron spinresonance spectrometer where the sample under investigation is arrangedon a dish. The dish is held at its bottom by a long rod which, togetherwith the dish, is retained axially inside an insulated glass tube. Alaser beam can be directed upon the sample directly from the oppositeside of the glass tube. It is further possible with this arrangement topass liquid nitrogen through the tube in order to quench the sampleafter the latter has been heated up by the laser beam. The sampletemperature can be measured by means of a thermocouple whose outputsignal is indicated and supplied, via a measuring amplifier, to atemperature control which in its turn is connected to a power supplyunit of the laser.

Although it is possible with this known arrangement to heat up thesample without taking recourse to a carrier gas, the accurate adjustmentof the temperature requires considerable input in this case, too, andthe known device is neither intended, nor suited for recording nuclearmagnetic resonance spectra where the sample usually rotates in themagnetic field.

The DE Magazine "BRUKER REPORT 2/1988" describes on pages 9 to 11 anuclear magnetic resonance spectrometer which enables nuclear magneticresonance measurments to be carried out on samples at very hightemperatures of up to 1,000° Centigrade. In the case of this knownarrangement, the sample is freely supported at the lower end of a rotor.

The rotor is arranged to rotate in the magnetic field of asuperconductive magnet. A laser beam is directed upon the sample fromthe bottom of the sample head, through a suitable passage opening.

Although this arrangement indeed enables the sample to be heated up bymeans of a laser beam during nuclear magnetic resonance measurementsperformed on a sample which rotates about the vertical axis, temperaturecontrol can be implemented only with considerable difficulty.

Another nuclear magnetic resonance spectrometer has been known fromFR-OS 1 628 214. In this case, the sample is arranged in the samplespace of a magnet, in a fixed vertical-axis sample container. Heating-upis implemented in the case of this known spectrometer also by means of alaser beam which is introduced from the bottom of the sample head, butwhich in this case impinges upon the sample container so that the sampleis heated up only indirectly.

In the case of this known arrangement, the temperature of the samplecontainer, with the sample contained therein, is adjusted to between400° Centigrade and 1,000° Centigrade by varying the intensity of thelaser beam. To this end, a pyrometer is provided for sensing the heatradiated from the upside of the sample container, for processing thevalues so recorded, and for supplying them to a temperature control.

It appears that this known arrangement is likewise relativelycomplicated, as regards the adjustment of the temperature of the sample,and in addition the space above the sample container is also employed inthe case of this known spectrometer--a fact which may lead to practicaldifficulties because this space usually is needed for accommodating themechanical mounting elements for the sample head.

All the known arrangements where the sample, or a sample container, isheated up with the aid of a laser beam, are in addition connected withthe following substantial disadvantage:

When the laser beam is directed upon the sample or the sample container,part of the irradiated light is indeed absorbed and converted to heat,but another part is reflected so that secondary light will fall uponthose components of the sample head which surround the sample containeror the sample as such. This leads to extremely undesirable heating-up ofthe surrounding components, in particular the RF coil and, consequently,to losses in the signal-to-noise ratio. In addition, heating-up of theair cushions may result in instable rotation conditions.

Now, it is the object of the present invention to improve a nuclearmagnetic resonance spectrometer and/or a method of the type describedabove so that the before-mentioned disadvantages will be avoided.

According to the nuclear magnetic resonance spectrometer mentioned atthe outset, this object is achieved according to the invention by alaser whose laser beam is directed upon a surface of the rotor, thelaser beam being adjustable in intensity by means of a control device.

The nuclear magnetic resonance spectrometer according to the inventiontherefore provides the advantage that when the sample is rotated at the"magic angle", measurements can be carried out on samples at hightemperatures, without any need for a carrier fluid, and without anycomponents in the neighborhood of the sample getting heated up more thannecessary.

According to the nuclear magnetic resonance spectrometer mentioned inthe second place, the object underlying the present invention isachieved by the fact that the surface is provided in a cavity of thesample holder, the cavity having only a single access opening. Thenuclear magnetic resonance spectrometer, therefore, offers the advantagethat while the sample container is heated up in the known manner bymeans of a laser beam, the energy of the laser beam is utilized almostfully for heating up the sample container and, thus, the sample, whereasthe surrounding components are not exposed to direct heating-up by thelaser beam.

Finally, the before-mentioned method achieves the object underlying thepresent invention by the following procedural steps:

Irradiating a laser beam upon a sample container arranged in a sampleholder of a nuclear magnetic resonance spectrometer;

varying the intensity of the laser beam and measuring the temperature ofthe sample container as a function of the intensity of the laser beam;

introducing a sample into the sample container; and

adjusting the temperature of the sample by presetting the intensity ofthe laser.

The before-mentioned procedural steps make appear still anotheressential advantage of the invention: The arrangement according to theinvention is the first to provide a simple possibility to calibrate thetemperature control, by directing the laser beam upon a surface of thesample container and/or the rotor which surface is defined as regardsits light-absorption properties. One then only has to determine, by asingle calibration, the variation of the temperature of the samplecontainer in response to the intensity of the laser beam, and is then ina position to adjust the sample temperature for subsequent tests in asimple way, by presetting the intensity of the laser beam. It goeswithout saying that to this end the heat capacity per unit volume shouldconveniently be selected to be much greater than the mean variance ofthe heat capacities per unit volume of different samples so that thethermal responsivity will vary only insignificantly when the sample isintroduced into the sample container.

The desirable degree of definition of the light-absorption properties isachieved most effectively by the use of ceramic materials. In certainpreferred embodiments of the invention, therefore, a ceramic surface isprovided for irradiating the laser beam, and preferably the entiresample container and/or the entire rotor consists for this purpose of aceramic material.

Ceramic materials generally have good absorption properties for the CO₂laser wavelengths technically available. In addition, only slighttemperature gradients are encountered, due to the generally favorableheat conductivity of ceramic materials, at the envisaged temperatures.

Other advantages of the invention will appear from the specification andthe attached drawing. It is understood that the features that have beendescribed before and will be explained hereafter may be used not only inthe described combinations, but also in any other combination, orindividually, without leaving the scope and intent of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWING

One embodiment of the invention will now be described in more detailwith reference to the drawing. The sole FIGURE shows a schematized,partially sectional side view of one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Regarding now the FIGURE, a sample head of a nuclear magnetic resonancespectrometer of conventional design is indicated generally by referencenumeral 10. The sample head 10 is delimited to the outside by a tube 11,which is indicated only schematically and whose longitudinal axisdefines at the same time a first axis 12 extending in the direction ofthe constant magnetic field H₀ of a magnet system which is not depictedin the figure for the sake of clarity.

In order to enable material properties of a sample 19 to be measured bynuclear magnetic resonance, the sample 19 is arranged in a sample holderindicated generally by 20. The sample holder 20 is designed to extendalong a second axis 21 enclosing with the first axis 12 an angle of54.7°, known as the "magic angle". It is, however, understood that theinvention is not limited to the indicated angles, but that other anglescan be selected without departing from the scope of the presentinvention.

The sample holder 20 comprises stationary bearings 22, indicated onlyschematically in the figure, which serve to rotatably support a rotor 23in the direction of the second axis 21.

The rotor 23 consists substantially of a small cylindrical tube 24 madefrom a ceramic material and enclosing an inner space 25 which isintended to receive the sample 19. The tube 24 is closed by a lid 26.

The circumference of the lid 26 is provided with engraved portions 27configured in the manner of turbine blades. The engraved portions 27 arearranged in the neighborhood of nozzles 28 which serve in the known wayfor directing a gas flow upon the engraved portions 27 in thecircumferential direction, in order to set the rotor 23 into rotation,at a revolution frequency of some KHz.

Further, there is provided an RF coil, indicated by 29, by means ofwhich the RF magnetic field required for nuclear magnetic resonancemeasurements can be irradiated upon the sample 19.

As can be clearly seen in the figure, a bottom 30 of the tube 24delimits a chamber 31 which, preferably, is formed integrally with thetube 24 and which also consists of a ceramic material.

The chamber 31 encloses a cavity 32. The cavity 32 communicates with theoutside via a single opening 33. Reference numeral 34 indicates an innersurface of the cavity 32. The surface 34 may be provided with a suitablelight-absorbing lining 35, or may be roughened.

Below the sample holder 20, there can be seen a laser 40, for example aCO₂ laser having a wavelength in the range of 10 μm and a maximum poweroutput of between 10 and 100 W.

The laser 40 can be adjusted in intensity by means of a control unit 41which may be integrated in the laser 40.

The laser 40 emits a laser beam 42, which is deflected by means ofmirrors 43, 44 in order to enter the cavity 32 through the opening 33,and to impinge upon the surface 34.

The figure shows that when the laser beam 42 hits upon the surface 34,it is reflected, and the reflected light rays impinge upon areas of thesurface 43, but are prevented from leaving the chamber 31, the latterbeing closed on all sides except for the opening 33, as has beendescribed further above. It is now possible, by selecting the surface34, or a lining 35, in a suitable way, to give the surface 34particularly diffuse reflection properties in order to prevent evensmall portions of the irradiated laser light from escaping through theopening 33. The cross-section of the opening 33 is of course selected assmall as possible; preferably, the opening 33 is just as big asnecessary in view of the cross-section of the laser beam 42, which ispreferably focused upon the opening.

Finally, one can see an infrared diode 50 which is arranged near therotor 23 and which is connected to the control unit 41 via a line 51.

The arrangement operates as follows:

In order to calibrate the temperature control system for the sample 19,the rotor 23 is preferably designed in such a way that its heat capacityper unit volume is considerably greater than the variance of the heatcapacities per unit volume of the samples 19 to be measured. The rotor23 is then introduced into the sample holder 20 either in emptycondition or filled with a sample of average properties, and set intorotation by introducing propellant gas through the nozzles 28.

One then switches on the laser 40, and the laser beam 42 is directed,via the mirrors 43, 44, into the opening 33 which, due to its axialarrangement, provides constant access to the cavity 32 during rotationof the rotor 33.

Now, the intensity of the laser beam 42 is varied by adjusting thecontrol unit 41, while measuring at the same time for the purposes ofthis calibration the temperature to which the rotor 23 is heated upunder the effect of the laser beam 42, using for this purpose theinfrared diode 50 which, preferably, is focused upon the rotor 23.

At the end of this procedure, a log will be available where thetemperature of the rotor and, thus, the temperature of the sample isrecorded as a function of the laser output or the setting of the controlunit 41, respectively.

If practical measurements are to be carried out later, it is then onlynecessary to set the control unit 41 to the respective calibrated valuewhich corresponds to a given intensity of the laser beam 42 and, thus,to a defined temperature of the rotor 23 and the sample 19.

It has been mentioned before that, generally, the arrangement describedabove is independent of the angle at which the rotor 23 is inclinedrelative to the first axis 12 of the constant magnetic field H₀, be itthe "magic angle" or any other angle. So, it can be imagined, forexample, to arrange the rotor 23 coaxially relative to the first axis12, as usual for conventional nuclear magnetic resonance measurements,because in this case, too, the sample 19 can be heated up in acontrolled way, through irradiation of the laser beam 42 into the cavity32, and this also during rotation of the rotor 23.

We claim:
 1. A nuclear magnetic resonance spectrometer,comprising:sample head means having a first axis arranged parallel to aconstant magnetic field and including a sample holder means; rotor meansfor receiving a sample under investigation, said rotor means beingarranged within said sample holder means and being rotatable about asecond axis inclined at an acute angle with respect to said first axis,said rotor means having a surface forming a cavity having only oneaccess opening; laser means for emitting a laser beam of controlledintensity; means for directing said laser beam through said accessopening and onto said surface of said rotor means for heating up saidrotor means; and control means connected to said laser means for settingsaid laser beam intensity.
 2. The spectrometer of claim 1, wherein saidacute angle is 54.7°.
 3. The spectrometer of claim 1, wherein saidsurface is a ceramic surface.
 4. The spectrometer of claim 3, whereinsaid rotor means consists essentially of a ceramic material.
 5. Thespectrometer of claim 1, wherein said surface is provided with alight-absorbing lining.
 6. The spectrometer of claim 1, wherein saidsurface is roughened.
 7. The spectrometer of claim 1, wherein saidaccess opening has a cross-section corresponding essentially to across-section of said laser beam.
 8. The spectrometer of claim 1,wherein said access opening has a cross-section correspondingessentially to a cross-section of a focal point of said laser beam. 9.The spectrometer of claim 1, wherein said control means is connected toa temperature sensor arranged on said sample holder means.
 10. Thespectrometer of claim 9, wherein said temperature sensor is an infrareddiode detecting temperature of said rotor means surface.
 11. A nuclearmagnetic resonance spectrometer comprising:a sample head having a firstaxis arranged parallel to a constant magnetic field and includingasample holder means including rotor means rotatable about a second axisdisposed at an acute angle with respect to said first axis, said rotormeans having a first cavity formed therein for receiving a sample underinvestigation and a second cavity formed therein having an interiorsurface accessible through an access opening, and means for rotatingsaid rotor means about said second axis; laser means for emitting alaser beam; means for directing said laser beam through said accessopening and onto said interior surface for heating said rotor means; andcontrol means for monitoring the temperature of said rotor means and forcontrolling the intensity of said laser beam.
 12. The spectrometer ofclaim 11, wherein said access opening has a cross-section correspondingessentially to a cross-section of said laser beam.
 13. The spectrometerof claim 11, wherein said access opening has a cross-sectioncorresponding essentially to a cross-section of a focal point of saidlaser beam.
 14. The spectrometer of claim 11, wherein said surface isprovided with a light-absorbing lining.
 15. The spectrometer of claim11, wherein said surface is roughened.
 16. The spectrometer of claim 11,wherein said surface is a ceramic surface.
 17. The spectrometer of claim16, wherein said rotor means is essentially comprised of a ceramicmaterial.
 18. A method for measuring nuclear magnetic resonance of asample under investigation within a constant magnetic field, whereinsaid sample is heated up during said measuring to a temperature aboveambient temperature, by means of a laser beam, the method comprising thesteps of:irradiating a laser beam of predetermined intensity upon asample container arranged in a sample holder of said nuclear magneticresonance spectrometer; varying said laser beam intensity and measuringtemperature of said sample container as a function of said laser beamintensity; introducing a sample under investigation into said samplecontainer; and adjusting said temperature of said sample underinvestigation by setting said laser beam intensity accordingly.
 19. Themethod of claim 18, comprising the further steps of:arranging saidsample under investigation along an axis being inclined by an angle of54.7° with respect to said constant magnetic field; and rotating saidsample under investigation about said axis.
 20. A method for measuringnuclear magnetic resonance of a sample under investigation within aconstant magnetic field wherein said sample is heated up during saidmeasuring to a temperature above ambient temperature by means of a laserbeam, the method comprising the steps of:introducing a sample underinvestigation into a sample container; disposing said sample containerwithin said constant magnetic field; rotating said sample container;irradiating said sample container with a laser beam to influence thetemperature thereof; measuring the temperature of said sample container;and varying the intensity of said laser beam as a function of themeasured temperature to control the temperature of said sample.
 21. Amethod for measuring nuclear magnetic resonance as recited in claim 20wherein the sample container is rotated about an axis oriented at anangle of substantially 54.7% with respect to the direction of saidconstant magnetic fields.